Keep @JavascriptInterface methods in proguard.
We need to keep methods that are tagged @JavascriptInterface as these
are discovered by reflection at runtime and used to create JS bindings.
Stripping these was breaking ChromeVox; works with this change.
Bug: 10766751
Change-Id: I1c0b9a855d99c812bd4a3f05e6373c5b2fe045a4
(cherry picked from commit d32e258033d74fb551442b10547215de443be05c)